Flat-panel displays such as liquid-crystal displays (LCDs) or organic light emitting diode (OLED) displays are useful in a wide variety of applications under a wide variety of environmental conditions. When viewed in a dark environment (little ambient radiation), such displays need not be as bright as when viewed in a lighter environment (more ambient radiation). If the display light output is adjusted periodically to compensate for ambient light conditions, the display can maintain a constant relative brightness with respect to the ambient illumination even if the ambient illumination changes. In a bright environment, this will increase display brightness to improve visibility. In a dark environment, this will increase display device lifetime and reduce power usage by reducing unnecessary display brightness.
The use of photosensors with displays to detect ambient light and adjusting the brightness of the display in response to ambient illumination is known. Efficient silicon photosensors are available and generally provide a current proportional to the light incident on the sensor. These photosensors are constructed on silicon substrates and may have a wide dynamic range. Such sensors can be combined with displays to provide ambient sensing. For example, see JP 2002-297096-A, which describes a circuit for providing ambient compensation to an electro-luminescent display. However, as implemented, the sensor is separate from the display and senses the light at a single point. This increases the cost, number of components, and size of the device and does not directly measure the light incident on the display itself.
It is known to integrate a light sensor on an active-matrix display device for the purpose of sensing light emitted from the display device itself. See, for example, U.S. Pat. No. 6,489,631 issued Dec. 3, 2002 to Young et al., which describes a display having integrated photosensors for sensing light emitted by a light emitting element of the display. There is no disclosure of the use of such photosensors for detecting ambient light, however, and the arrangement of the sensor coupled with a light emitter limits the size of the photosensor and its ability to sense ambient light.
When providing ambient compensation to a display, it is important that the light-sensing device provide a signal having a wide dynamic range representative of the ambient illumination. The human visual system can effectively detect light from very dark ambient conditions of only a few photons to very bright outdoor conditions greater than 75,000 lux. However, tests conducted by applicant demonstrate that photosensors constructed on flat-panel displays using thin-film technology do not have the efficiency of photosensors constructed on silicon substrates and do not have the sensitivity necessary to provide a signal representative of lower light levels, for example <100 cd/m2, where displays are often used. Nor do they have the dynamic range necessary to accommodate the range of the human visual system.
Typical circuits used in combination with photosensors such as, for example, photodiodes or phototransistors, rely upon either direct measurement of the current produced or an accumulation of the current and measurement of the charge accumulated. In the first case, the current produced by a typical thin-film photosensor in a flat-panel display is very small, on the order of femto-Amps, making the reliable detection of the current very difficult. In the second case, the charge is typically converted to a voltage signal that is sensed with an analog-to-digital converter (ADC). For example, U.S. Pat. No. 6,740,860 entitled “Photodetector, photosensing position detector, coordinate input device, coordinate input/output apparatus, and photodetection method” issued 2004 May 25 describes a circuit using photosensors and ADC circuits. However, analog-to-digital converter circuits are complex and expensive and difficult to fabricate on a glass substrate such as is commonly found in a flat-panel display.
There is a need therefore for an improved photosensor circuit for the detection of light, particularly for detection of ambient light incident on an active-matrix flat-panel display.